This research proposal is designed to illuminate the developmental mechanisms underlying the formation of specific connections between motoneurons and muscles. These studies address the questions of how growing motor axons in the axolotl (Ambystoma mexicanum) arrive at their appropriate targets and whether they can distinguish between their own targets and those appropriate to different neurons. Experiments will involve microsurgury, electrophysiology, electron microscopy and several histological methods including fluorescence microscopy. The project has the following aims: 1. to describe the segmental innervation of axolotl hindlimb extensor muscles. This will be done by recording from muscles and labeling motoneuron pools; 2. To describe the development of the adult innervation pattern, in particular the contribution of synapse elimination to refining the embryonic pattern; 3. to study the behavior of motor axons after microsurgery on the developing spinal cord or lumbar plexus in axolotl embryos; and 4. to study the segmental innervation of adult muscles tranplanted to new positions. The axolotl nerve-muscles system has been chosen because a) it is accessible both for recording from muscles and stimulation of their nerves; b) mature axololts recover normal limb function after damage to peripheral nerves, suggesting that some of the mechanisms responsible for establishing motor projections during embryogenesis persist and might be amenable to study in adult life; c) axolotl embryos survive microsurgery on both developing spinal cord and muscle tissue, permitting experiments so far impossible with mammalian embryos; and d) there is an expanding library of mutants useful for future experiments. Through this work I hope to learn how synaptic connections are made selectively between neurons and certain muscles but not others. Similar selectivity probably underlies the orderly assembly of the nervous system during embryogenesis. I hope further that results from these experiments will help identify the location and nature of the molecular cues that are probably involved in axon guidance and selective synapse formation.